1. Field of the Invention
The present invention relates to a process for refining a heat deteriorative compound, contained in a multi-component liquid mixture, by distillation. More particularly, the present invention relates to a process for refining a heat deteriorative compound, for example, hydroquinone or catechol which are consumed by thermal decomposition, thermal modification or thermal polymerization when heated, and contained in a multi-component liquid mixture, by a specific distillation procedure by which the multi-component liquid mixture is separated, at a temperature lower than the heat deterioration-starting temperature of the heat-deteriorative compound, into a liquid fraction containing a higher boiling temperature component including the heat-deteriorative compound and a vapor fraction containing a lower boiling temperature component which is substantially free from the heat-deteriorative compound and is removed from the multi-component liquid mixture.
2. Description of the Related Art
It is known that a reduced pressure (vacuum) distillation apparatus equipped with a liquid film-falling reboiler attached to a distillation column is advantageous in that the liquid mixture can be distilled even under a reduced pressure, while keeping the temperature of a liquid fraction accumulated in a bottom portion of the distillation column at a relatively low level, and is thus widely utilized for various distillations which must be carried out at a relatively low distillation temperature.
When the conventional distillation apparatus comprising a distillation column, which may be a reduced pressure distillation column and a liquid film-falling reboiler attached to the distillation column, is used to distil, optionally under a reduced pressure, a liquid mixture containing a heat-deteriorative compound, and a liquid fraction generated in the distillation column is fed into the reboiler and falls in the form of liquid films through a plurality of heat-conductive pipes of the reboiler, such a disadvantageous phenomenon that the falling liquid films are broken on portions of the inner surfaces of the heat conductive pipes, and thus the portions of the inner surfaces of the heat conductive pipes are directly exposed to the air atmosphere without being covered by the falling liquid film of the liquid fraction, may occur. When the phenomenon occurs, the portions of the inner surfaces not covered by the falling liquid films are locally over-heated to a higher temperature than the temperature of the other portions of the inner surfaces, and thus portions of the falling liquid films located in boundaries between the liquid film-covered portions and non-covered portions of the inner surfaces of the heat conductive pipes are also locally overheated to a higher temperature than that of the other portions. Accordingly, the heat-deteriorative compound contained in the locally over-heated portions of the falling liquid films is thermally deteriorated to a great extent. Therefore, the heat deteriorative compound contained in the liquid mixture is converted to a high boiling temperature substance (for example, polymer), a thermally decomposed product or a thermally modified product, and consumed in a high proportion during the distillation procedure in the reboiler. In a certain case in which the resultant high boiling temperature substance, for example, a polymeric substance, deposits on and adheres to the inner surfaces of the heat conductive pipes, to form scale, the resultant scale causes the falling of the liquid film along the inner surfaces of the heat conductive pipes to be obstructed and finally stopped, and thus the distilling procedure must be stopped after a short time.
For example, in a conventional process for producing an alkyl ether of an aromatic dihydroxyl compound by an etherifying reaction of an aromatic dihydroxyl compound, for example, hydroquinone or catechol with a lower alkyl alcohol, for example, methyl alcohol or ethyl alcohol, the reaction product liquid comprising a lower boiling temperature component containing the non-reacted lower alkyl alcohol and a higher boiling temperature component containing the non-reacted aromatic dihydroxyl compound and the resultant alkyl ether of the aromatic dihydroxyl compound is distilled by a conventional distillation procedure at a high temperature to collect the target reaction product and to recover the non-reacted compounds. In this conventional distillation procedure, the aromatic dihydroxyl compound which exhibits a high thermal deterioration property is concentrated and thermally deteriorated at the high temperature. The thermal deterioration results in a disadvantageous consumption or loss of the aromatic dihydroxyl compound. Also, in a certain case, the heat-deterioration product deposits and adheres to an inner surface of a heater (or reboiler) of the distillation apparatus and the continuous distilling procedure over a long time is obstructed.
Namely, in the process for producing the alkyl ether of the aromatic dihydroxyl compound, no specific method of refining the target aromatic dihydroxyl compound and of recovering the non-reacted lower alkyl alcohol and aromatic dihydorxyl compound, without heat deterioration of the aromatic dihydroxyl compound has yet been concretely provided.
As mentioned above, the reduced pressure (vacuum) distillation apparatus including the liquid film-falling reboiler attached to a distillation column is advantageous in that the liquid fraction accumulated in the bottom portion of the distillation column can be distilled even under a reduced pressure without a need of heating to a high temperature. Therefore, the reduced pressure distillation apparatus can be used for a distillation procedure for a liquid mixture containing the alkyl ether of aromatic dihydroxyl compound produced by the reaction of the alkyl alcohol with the aromatic dihydroxyl compound.
However, when the distillation procedure is carried out under conventional conditions, the liquid films falling along the inner surfaces of the heat conductive pipes of the reboiler are frequently broken and thus the heat-deteriorative compound contained in the liquid film is thermally deteriorated as mentioned hereinbefore. No specific means for preventing the breakage of the falling liquid films on the inner surfaces of the heat conductive pipes of the reboiler has been known.
An object of the present invention is to provide a process for refining a heat-deteriorative compound contained in a multi-component liquid mixture by using a distillation column and a liquid film falling reboiler attached to the distillation column and having a plurality of vertical heat-conductive pipes through which a liquid fraction generated from the liquid mixture in the distillation column and containing the heat-deteriorative compound falls in the form of liquid films along the inner surface of the vertical pipes while being heated and evaporated, in which process, local breakages of the liquid films on the inner surface of the vertical pipes can be prevented, to protect the heat-deteriorative compound in the liquid fraction from heat-deterioration in the reboiler, and thus the heat-deteriorative compound contained in the liquid mixture can be continuously concentrated and refined with a high stability and with a high yield over a long time period.
The above-mentioned object can be attained by the process of the present invention for refining a heat-deteriorative compound, contained in a multi-component liquid mixture, by distillation, which comprises the steps of:
(1) feeding a multi-component liquid mixture comprising a lower boiling temperature component and a higher boiling temperature component containing a heat-deteriorative compound, into a distillation column having a top outlet located in a top portion of the distillation column and a bottom outlet located in a bottom portion of the distillation column, the bottom outlet being connected to a liquid film-failing reboiler having a plurality of heat-conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes:
(2) distilling the multi-component liquid mixture in the distillation column to generate a vapor fraction comprising the lower boiling temperature component and a liquid fraction comprising the higher boiling temperature component, in such a manner that (A) the liquid fraction is withdrawn through the bottom outlet of the distillation column and introduced into the reboiler to allow the introduced liquid fraction to fall in the form of films along inner surfaces of the vertical pipes; (B) the falling liquid fraction through the vertical pipes is heated by the heating medium passing through the heating chamber at a temperature lower than the heat-deterioration-starting temperature of the heat-deteriorative compound but sufficient to evaporate the liquid fraction, to evaporate a portion of the falling liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the reboiler, per one pass of the liquid fraction through the reboiler; (C) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the reboiler and returned into the distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the distillation column; and (D) the evaporated portion of the returned liquid fraction heat-exchanges to the multi-component liquid mixture fed into the distillation column to evaporate the lower boiling temperature component;
(3) delivering the resultant vapor fraction comprising the lower boiling temperature component through the top outlet of the distillation column, while allowing the resultant liquid fraction comprising the higher boiling temperature component to be accumulated in the bottom portion of the distillation column; and
(4) recovering a portion of the liquid fraction accumulated in the bottom portion of the distillation column and comprising the higher boiling temperature component containing the heat-deteriorative compound from the bottom portion of the distillation column.
When the higher boiling temperature component contained in the liquid fraction recovered in the recovery step (4) contains, in addition to the heat-deteriorative compound, at least one organic compound having a boiling temperature lower than that of the heat-deteriorative compound, the process of the present invention further comprises the steps of:
(5) feeding the recovered liquid fraction into an additional distillation column having a top outlet located in a top portion of the additional distillation column and a bottom outlet located in a bottom portion of the additional distillation column, the bottom outlet being connected to an additional liquid film-falling reboiler having a plurality of heat-conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes;
(6) distilling the fed liquid fraction in the additional distillation column to generate a liquid fraction comprising the heat-deteriorative compound and a vapor fraction comprising the organic compound having a lower boiling temperature than that of the heat-deteriorative compound, in such a manner that (E) the liquid fraction is withdrawn through the bottom outlet of the additional distillation column and introduced into the additional reboiler to allow the introduced liquid fraction to fall in the form of films along inner surfaces of the vertical pipes of the additional reboiler; (F) the falling liquid fraction through the vertical pipes is heated by the heating medium passing through the heating chamber at a temperature lower than the heat deterioration-starting temperature of the heat-deteriorative compound but sufficient to evaporate the falling liquid fraction, to evaporate a portion of the falling liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the additional reboiler, per one pass of the liquid fraction through the additional reboiler; (G) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the additional reboiler and returned into the additional distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the additional distillation column; and (H) the returned evaporated portion in the additional distillation column heat-exchanges to the recovered liquid fraction fed into the additional distillation column, to evaporate the organic compound having the lower boiling temperature than that of the heat-deteriorative compound;
(7) delivering the resultant vapor fraction comprising the organic compound having the lower boiling temperature than that of the heat-deteriorative compound through the top outlet of the additional distillation column, while allowing the resultant liquid fraction comprising the heat-deteriorative compound to be accumulated in the bottom portion of the additional distillation column; and
(8) recovering a portion of the liquid fraction, accumulated in the bottom portion of the additional distillation column and comprising the heat-deteriorative compound, through the bottom outlet of the additional distillation column.
Further, when the liquid fraction recovered from the additional distillation column in step (8) contains, in addition to the heat-deteriorative compound, at least one organic compound having a higher boiling temperature than that of the heat-deteriorative compound, the process of the present invention further comprising the steps of:
(9) feeding the recovered liquid fraction from the additional distillation column into a further additional distillation column having a top outlet located in a top portion of the further additional distillation column and a bottom outlet located in a bottom portion of the further additional distillation column, the bottom outlet being connected to a further additional liquid film-falling reboiler having a plurality of heat conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes;
(10) distilling the fed liquid fraction in the further additional distillation column to generate a vapor fraction comprising the heat-deteriorative compound and a liquid fraction comprising the organic compound having the higher boiling temperature than that of the heat-deteriorative compound, in such a manner that (I) the liquid fraction in the further additional distillation column is withdrawn through the bottom outlet of the further additional distillation column and introduced into the further additional reboiler to allow the introduced liquid fraction to fall in the form of films along inner surfaces of the vertical pipes of the further additional reboiler; (J) the falling liquid fraction through the vertical pipes is heated by the heating medium, passing through the heating chamber, at a temperature lower than the heat-deterioration-starting temperature of the heat-deteriorative compound but sufficient to evaporate the falling liquid fraction, to evaporate a portion of the failing liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the further additional reboiler, per one pass of the liquid fraction through the further additional reboiler; (K) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the further additional reboiler and are returned into the further additional distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the further additional distillation column; and (L) the returned evaporated portion in the further additional distillation column heat-exchanges to the recovered liquid fraction into the further additional distillation column, to evaporate the heat-deteriorative compound;
(11) recovering the resultant vapor containing the heat-deteriorative compound through the top outlet of the further additional distillation column, while allowing the resultant liquid fraction comprising the organic compound having higher boiling temperature than that of the heat-deteriorative compound to be accumulated in the bottom portion of the further additional distillation column; and
(12) discharging a portion of the liquid fraction accumulated in the bottom portion through the bottom outlet of the further additional distillation column.
In an embodiment of the process of the present invention,
(1) a multi-component liquid mixture comprising a lower boiling temperature component comprising at least one alkyl alcohol having 1 to 20 carbon atoms and a higher boiling temperature component comprising a heat-deteriorative aromatic compound having two or more hydroxyl groups and at least one alkyl ether of the aromatic compound having two or more hydroxyl groups, is fed into a first distillation column having a top outlet located in a top portion of the first distillation column and a bottom outlet located a bottom portion of the first distillation column, the bottom outlet being connected to a first liquid film-falling reboiler having a plurality of heat-conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes;
(2) the fed multi-component liquid mixture is distilled in the first distillation column to generate a vapor fraction comprising the lower boiling temperature component and a liquid fraction comprising the higher boiling temperature component, in such a manner that (A) the liquid fraction is withdrawn through the bottom outlet of the first distillation column and introduced into the first reboiler in which the introduced liquid fraction falls in the form of films along the inner surfaces of the vertical pipes; (B) the falling liquid fraction is heated by the heating medium passing through the heating chamber at a temperature lower than the heat-deterioration-starting temperature of the aromatic compound having two or more hydroxyl groups but sufficient to evaporate the falling liquid fraction, to evaporate a portion of the falling liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the first reboiler, per one pass of the liquid fraction through the first reboiler; (C) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the first reboiler and returned into the first distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the first distillation column; and (D) the evaporated portion of the returned liquid fraction heat-exchanges with the multi-component liquid mixture fed into the first distillation column to evaporate the lower boiling temperature component;
(3) the resultant vapor fraction comprising the lower boiling temperature component is delivered through the top outlet of the first distillation column, while allowing the resultant liquid fraction comprising the higher boiling temperature component to accumulate in the bottom portion of the first distillation column;
(4) a potion of the liquid fraction accumulated in the bottom portion of the first distillation column and comprising the higher boiling temperature component containing the heat-deteriorative aromatic di- or more hydroxyl compound is recovered through the bottom outlet of the first distillation column;
(5) the recovered liquid fraction, which comprises the higher boiling temperature component comprising the heat-deteriorative aromatic di- or more hydroxyl compound and the alkylether of the heat-deteriorative aromatic compound having a boiling temperature lower than that of the heat-deteriorative compound, is fed into a second distillation column having a top outlet located in a top portion of the second distillation column and a bottom outlet located in a bottom portion of the second distillation column, the bottom outlet being connected to a second liquid film-falling reboiler having a plurality of heat-conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes;
(6) the fed liquid fraction is distilled in the second distillation column to generate a liquid fraction comprising the heat-deteriorative aromatic compound and a vapor fraction comprising the alkylether of the heat-deteriorative aromatic compound having a lower boiling temperature than that of the heat-deteriorative aromatic compound, in such a manner that (E) the liquid fraction in the second distillation column is withdrawn through the bottom outlet of the second distillation column and introduced into the second reboiler to allow the introduced liquid fraction to fall in the form of liquid films along inner surfaces of the vertical pipes of the second reboiler; (F) the liquid fraction falling through the vertical pipes is heated by the heating medium passing through the heating chamber at a temperature lower than the heat-deterioration-starting temperature of the heat-deteriorative aromatic compound but sufficient to evaporate the falling liquid fraction, to evaporate a portion of the falling liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the second reboiler, per one pass of the liquid fraction through the second reboiler; (G) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the second reboiler and returned into the second distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the second distillation column; and (H) the returned evaporated portion in the second distillation column heat-exchanges to the recovered liquid fraction fed into the second distillation column, to evaporate the alkylether of the heat-deteriorative aromatic compound having the lower boiling temperature than that of the heat-deteriorative compound;
(7) the resultant vapor fraction comprising the alkylether of the heat-deteriorative aromatic compound having the lower boiling temperature than that of the heat-deteriorative aromatic compound is delivered through the top outlet of the second distillation column, while allowing the resultant liquid fraction comprising the heat-deteriorative compound to be accumulated in the bottom portion of the second distillation column; and
(8) a portion of the liquid fraction accumulated in the bottom portion of the second distillation column and comprising the heat-deteriorative aromatic compound is recovered through the bottom outlet of the second distillation column.
In the embodiment of the present invention, when the liquid fraction recovered from the second distillation column contains, in addition to the heat-deteriorative compound, at least one organic compound having a higher boiling temperature than that of the heat-deteriorative aromatic compound, the recovered liquid fraction is further refined by the procedure in which;
(9) the recovered liquid fraction from the second distillation column is fed into a third distillation column having a top outlet located in a top portion of the third distillation column and a bottom outlet located in a bottom portion of the third distillation column, the bottom outlet being connected to a third liquid film-falling reboiler having a plurality of heat-conductive vertical pipes spaced from each other and arranged in a heating chamber through which a heating medium passes;
(10) the fed liquid fraction is distilled in the third distillation column to generate a vapor fraction comprising the heat-deteriorative compound and a liquid fraction comprising the organic compound having the higher boiling temperature than that of the heat-deteriorative compound, in such a manner that (I) the liquid fraction in the third distillation column is withdrawn through the bottom outlet of the third distillation column and introduced into the third reboiler to allow the introduced liquid fraction to fall in the form of films along inner surfaces of the vertical pipes of the third reboiler; (J) the falling liquid fraction through the vertical pipes is heated by the heating medium passing through the heating chamber at a temperature lower than the heat-deterioration-starting temperature of the heat-deteriorative compound but sufficient to evaporate the falling liquid fraction, to evaporate a portion of the falling liquid fraction in an evaporation amount in kg/hr corresponding to 1 to 15% by weight of the total amount in kg/hr of the liquid fraction introduced into the third reboiler, per one pass of the liquid fraction through the third reboiler; (K) the resultant evaporated and non-evaporated portions of the liquid fraction are withdrawn from the third reboiler and returned into the third distillation column through a returning inlet thereof located above the level of the liquid fraction accumulated in the bottom portion of the third distillation column; and (L) the returned evaporated portion in the third distillation column heat-exchanges to the recovered liquid fraction fed into the third distillation column, to evaporate the heat-deteriorative compound;
(11) the resultant vapor containing the heat-deteriorative compound is recovered through the top outlet of the third column, while allowing the resultant liquid fraction comprising the organic compound having higher boiling temperature than that of the heat-deteriorative compound to be accumulated in the bottom portion of the third distillation column; and
(12) a portion of the liquid fraction accumulated in the bottom portion of the third distillation column is discharged through the bottom outlet of the third distillation column.